Root resorption during fixed appliance therapy – mechanism, risk factors and prognosis

External inflammatory root resorption is an inevitable iatrogenic effect of orthodontic treatment. It results in root shortening, and occurs to some extent in all patients treated with fixed appliances. For the majority of patients the shortening is minimal (< 2 mm) and causes no long term clinical problems. However, for a small percentage of patients the resorption is more severe and generalized, and has implications for the long term health and longevity of the dentition (Figure 1). The variation in the severity of resorption cannot be accounted for by different forces or mechanics used alone. There appears to be a susceptibility to resorption amongst certain patients. Recent research has focused on identifying risk factors, so patients in the higher risk categories may be made aware of this as part of the informed consent process prior to treatment.

The prevalence reported in the literature varies widely. Resorption is a 3-D phenomenon, which is difficult to quantify accurately using traditional radiographic methods. Root shortening of over 2 mm has been reported in 10–18% of patients^1,2 by studies using OPT investigations. A study using histological techniques reported much higher figures of 93% of teeth showing signs of resorption following orthodontic force application for 4 weeks.³ Cone beam computed tomography has been demonstrated to be more sensitive in diagnosing resorption lesions⁴ and, using this technique, Lund et al reported root shortening of at least 1 mm in 94% of patients.⁵ In the majority of patients, resorption of less than 2 mm will be clinically insignificant, and may be accepted as an inevitable result of orthodontic treatment. Shortening of over 4 mm has been reported in approximately 6% of patients,⁵ and it is this cohort of patients who may have long term detrimental effects.